The present invention relates generally to fiber optic telecommunication systems. More specifically, the present invention relates to a fiber optic receiver assembly for use in a passive optical network that must receive, convert and sequence burst mode optical signals from a plurality of sources, wherein each signal being received has a different amplitude (power level) and relative timing as compared to adjacent bursts.
In an effort to increase the available bandwidth for the delivery of services such as streaming media and the like, telecommunication companies are slowly starting to convert their carrier networks from traditional copper wiring to passive optical network systems as part of Fiber To The Premise (FTTP) and Fiber To The Home (FTTH) communication and content delivery services. For example, a number of telecommunications service providers are now offering fiber optic Internet services and fiber optic television services. These passive optical networks are structured in a manner that is generally similar to the older copper wire networks. As can be seen in FIG. 1, a representative passive optical network can be seen to a plurality of communications lines 2 that extend outwardly to each of the individual service locations 4 (homes or businesses) from a central office 6 (CO) location. The CO 6 in turn serves to control, direct and monitor the transmission and receipt of the signals 8, 10, 12 traveling to and from each of the connected individual service locations 4.
In operation, the various signals from the individual service locations are connected to a single transmission fiber using a passive optical splitter. To allow the sharing of the fiber bandwidth in this manner each service location transfers a packet of data using a predetermined time slot. This type of data is typically called burst mode data. In other words, a plurality of subscribers utilizes one optical line in a time division multiplex manner, but a receiver on the line recognizes that each subscriber sends data in bursts at random times. The difficulty with this arrangement in a passive optical network is that received data or packets are different in both amplitude and phase from one another due to the differences in optical losses occurring on different transfer paths.
In this regard, one of the current technical issues being addressed in the implementation of the passive optical networking environment is the fact that there is a large variation in both the amplitude and the timing of the incoming signals 8, 10, 12 being received at the CO 6 from each of the individual service locations 4. For example, in a FTTH system, one service location 4 may be located 0.5 miles from the central office 6, while another service location 4 may be located 5.0 miles from the central office 6. In each of these service locations 4, the transmitters are essentially the same and therefore transmit their respective data signals 8, 10, 12 to the CO 6 using the same output power level. The difficulty arises as a result of the fact that, since optical signals degrade within a fiber optic cable over distance and pick up slight timing delays due to the distance traversed, the signal received 12 at the CO 6 from the closer service location has a timing delay and an amplitude that is larger than the amplitude of the signal received 10 at the CO 6 from the more distant location. These differences in amplitude and timing become a problem because the passive optical network systems are time division multiuplexed (TDM) systems, where the CO 6 receiver is constantly receiving timed bursts 8, 10, 12 in a random pattern from each of the different locations, one after another, with a signal spacing of tens of nano-seconds. As a result, the receivers must be able to quickly and accurately detect and convert all of the incoming signal bursts from the different locations, each having varying amplitudes and time delays, into valid data. Further, in order to correctly process each of the bursts, the CO receiver needs to predict the time delay associated with transmission traveling along each of these different fiber transmission paths.
In order to account for this variable delay, another important aspect to the operation of the passive optical network system is the “ranging” mode. In this mode, the CO box, which has a multitude of fiber optic transceivers within it that are each attached to the various transmission fibers, sends out a one/zero pattern continuously until it receives a signal-detect signal. Based on the receipt of a signal detect the box knows the fiber delay associated with the transceiver module on any given fiber and prepares the timing signal accordingly to interact with it. In the prior art arrangement as depicted at FIGS. 2 and 3, the system employs a detector 14 and a low pass filter 16 is used to determine the level of the input signal. This level is compared to a reference level 18, by comparator 20 and if the signal exceeds the reference level the signal detect is asserted. The input to this detector 20 can come from the output of the transimpedance amplifier 22 as shown in FIG. 2 or the output of the post amplifier 23 as shown in FIG. 3. Generally however the problem that is encountered in this mode is that the signal detect is often triggered prematurely by noise that is present in the system. When noise is encountered during the time that the transceiver is waiting for the return signal, the transimpedance amplifier and or the post amplifier may treat noise detected in the system as a valid data signal thereby creating an incorrect delay factor.
In view of the foregoing there is therefore a need for a transceiver that can be used in a passive optical networking environment that reduces the signal noise that is passed along to the signal detect monitor in order to reduce the number of false signal detect triggers. Further, there is a need for a transceiver that filters the ranging signal in a manner that only allows the returning ranging signal to reach the detector to generate a signal detect signal thereby eliminating false triggers and insuring correct delay factor calculation.